Field of the Invention
The present invention relates to an automatic focusing apparatus as well as a lens apparatus and an image pickup system including the automatic focusing apparatus, and particularly, to an automatic focusing apparatus with an automatic focus function of a contrast method and a phase difference method using a taken image.
Description of the Related Art
Conventionally, various automatic focus (AF) techniques in an image pickup apparatus, such as a camera and a video camera, are proposed. For example, phase difference AF of a TTL (Through The Lens) method is proposed, in which a separation unit is included in an optical path in an imaging optical system, and an in-focus state is detected by separated beams to perform AF control. External measure AF of a non-TTL method is also proposed, in which a beam of external light different from the beam in an imaging optical system is used. Contrast AF based on a so-called hill-climbing method is further proposed, in which an image signal output from an image pickup element is used to calculate a focal point evaluation value. Hybrid AF with a combination of the phase different AF, the external measure AF and the contrast AF is also proposed.
Among these, electric charge according to the beam from an object is accumulated on a photoelectric conversion element in a focal point detecting apparatus in the phase difference AF and the external measure AF. A two-image signal read from the photoelectric conversion element is used to perform correlation computation to calculate a deviation amount, i.e. a phase difference, of images. In this case, a degree of coincidence between two images is handled as a correlation evaluation value to calculate a target value to an in-focus point. In general, a phase difference with an extreme and maximum correlation evaluation value is set as a highly reliable target value. The target value is then converted to a target position of a focus lens based on a defocus amount to the in-focus point and distance information to the object, and the drive of the focus lens is controlled. The object distance can be directly obtained in the phase difference AF and the external measurement AF, and the in-focus determination can be quickly performed.
In the contrast AF, a high frequency component extracted from an image signal by a filter of a band required for the in-focus determination is extracted as a contrast AF evaluation value. The focal point is adjusted by moving and controlling the focus lens to maximize the contrast AF evaluation value. When an object is imaged, the position of the focus lens with the maximum contrast AF evaluation value is usually the in-focus point. In this way, the image signal obtained based on the output signal from the image pickup element that images the object is used to perform the in-focus determination in the contrast AF method, and highly accurate focusing is possible.
Various methods for controlling an accumulating operation of the focal point detecting sensor of the phase difference AF and the external measurement AF are proposed. For example, there is a control method of finishing the accumulating operation when the signal reaches a predetermined signal level based on AGC (Auto Gain Control) control. Another example includes a control method of finishing the accumulating operation when a predetermined maximum accumulating time period has passed even if the signal does not reach a predetermined signal level. Both of the two types of accumulation control methods may be used to handle a wide dynamic range of object luminance due to various imaging conditions.
AF that attains both the focus speed and the focus accuracy is enabled by forming the hybrid AF with a combination of the features of the AF methods described above. However, if the luminance or the contrast of the object is low, the accumulating operation of the phase difference AF method and the external measurement AF method may require a long time. Therefore, conventional examples for performing automatic focusing in the case where the luminance or the contrast of the object is low are proposed.
For example, in Japanese Patent Application Laid-Open No. 2009-048123, the phase difference AF is terminated, and the focusing operation is performed only by the contrast AF when it is determined that the object luminance or the contrast is low. In the case where the object luminance is low, the accumulation by the phase difference sensors requires a long time. Therefore, the processing time can be reduced by an amount of the accumulating time period by performing the focusing operation only by the contrast AF.
In Japanese Patent Application Laid-Open No. 2006-023653, the search range of the contrast AF is set based on the depth of field and the object luminance. When the depth of field is shallow and when the object luminance is high, the focal point can be detected even if the search range is narrowed down. The center point of the search range is set as the target position of the phase difference AF. According to the configuration, the drive of the focus lens is efficiently controlled by driving the focus by the phase difference AF outside of the search range and switching to focus driving by the contrast AF inside of the search range.
However, in the conventional techniques disclosed in the patent documents, the time required for focusing may be different depending on the imaging conditions. More specifically, in the conventional driving methods of focus lens, the AF method is different between when the contrast of the object is high and when the contrast of the object is low. In Japanese Patent Application Laid-Open No. 2009-048123, the focal point detection is attempted only by the contrast AF for the low contrast object, and the focusing time is long. In Japanese Patent Application Laid-Open No. 2006-023653, after switching to the contrast AF near the focus, the driving interval of the focus lens for sampling and calculating the contrast AF evaluation value is constant. Furthermore, the search range is wide when the contrast is low, and as a result, the focusing time is long.
As for the contrast AF evaluation value, the curve characteristics and the peak value of the contrast AF evaluation value until focusing are different between high contrast and low contrast. Particularly, the contrast AF evaluation value does not change much from the defocus state to the in-focus point when the contrast is low, and the prediction of the curve characteristics and the peak position is difficult. As a result, to prioritize the focus accuracy, the sampling interval of the focus lens position of the contrast AF evaluation value calculation needs to be reduced, and the focusing time is long.
An object of the present invention is to improve the focus lens driving method until focusing regardless of the level of the contrast of the object and to enable quick and highly accurate focusing compared to the conventional methods.